Dust Grains In Protoplanetary Disks Research Articles

Probing Dust Settling in Proto-planetary Disks with ALMA

Investigating the dynamical evolution of dust grains in proto-planetary disks is a key issue to understand how planets should form. We identify under which conditions dust settling can be constrained by high angular resolution ALMA observations at mm wavelengths, and which observational strategies are suited for such studies. We find out that an angular resolution better than or equal to » 0.1” (using 2.3 km baselines at 0.8mm) allows us to constrain the dust scale height and flaring index with sufficient precision to unambiguously distinguish between settled and non-settled disks, provided the inclination is close enough to edge-on (i > 75°). Ignoring dust settling and assuming hydrostatic equilibrium when analyzing such disks biase the derived dust temperature, the radial dependency of the dust emissivity index and the surface density distribution.

SHORT CIRCUITS IN THERMALLY IONIZED PLASMAS: A MECHANISM FOR INTERMITTENT HEATING OF PROTOPLANETARY DISKS

Many astrophysical systems of interest, including protoplanetary accretion disks, are made of turbu- lent magnetized gas with near solar metallicity. Thermal ionization of alkali metals in such gas exceeds non-thermal ionization when temperatures climb above roughly 1000 K. As a result, the conductiv- ity, proportional to the ionization fraction, gains a strong, positive dependence on temperature. In this paper, we demonstrate that this relation between the temperature and the conductivity triggers an exponential instability that acts similarly to an electrical short, where the increased conductivity concentrates the current and locally increases the Ohmic heating. This contrasts with the resistiv- ity increase expected in an ideal magnetic reconnection region. The instability acts to focus narrow current sheets into even narrower sheets with far higher currents and temparatures. We lay out the basic principles of this behavior in this paper using protoplanetary disks as our example host system, motivated by observations of chondritic meteorites and their ancestors, dust grains in protoplanetary disks, that reveal the existence of strong, frequent heating events that this instability could explain.

Revisiting the “radial-drift barrier” of planet formation and its relevance in observed protoplanetary discs

Context. To form metre-sized pre-planetesimals in protoplanetary discs, growing grains have to decouple from the gas before they are accreted onto the central star during their phase of fast radial migration and thus overcome the so-called "radial-drift barrier" (often inaccurately referred to as the "metre-size barrier"). Aims. To predict the outcome of the radial motion of dust grains in protoplanetary discs whose surface density and temperature follow power-law profiles, with exponent p and q respectively. We investigate both the Epstein and the Stokes drag regimes which govern the motion of the dust. Methods. We analytically integrate the equations of motion obtained from perturbation analysis. We compare these results with those from direct numerical integration of the equations of motion. Then, using data from observed discs, we predict the fate of dust grains in real discs. Results. When a dust grain reaches the inner regions of the disc, the acceleration due to the increase of the pressure gradient is counterbalanced by the increase of the gas drag. We find that most grains in the Epstein (resp. the Stokes) regime survive their radial migration if-p+q+1/2 \leq0 (resp. if q\leq 2/3). The majority of observed discs satisfies both-p+q+ 1/2 \leq0 and q\leq 2/3: a large fraction of both their small and large grains remain in the disc, for them the radial drift barrier does not exist.

ON THE EVOLUTION OF DUST MINERALOGY, FROM PROTOPLANETARY DISKS TO PLANETARY SYSTEMS

Mineralogical studies of silicate features emitted by dust grains in protoplanetary disks and solar system bodies can shed light on the progress of planet formation. The significant fraction of crystalline material in comets, chondritic meteorites, and interplanetary dust particles indicates a modification of the almost completely amorphous interstellar medium dust from which they formed. The production of crystalline silicates, thus, must happen in protoplanetary disks, where dust evolves to build planets and planetesimals. Different scenarios have been proposed, but it is still unclear how and when this happens. This paper presents dust grain mineralogy (composition, crystallinity, and grain size distribution) of a complete sample of protoplanetary disks in the young Serpens cluster. These results are compared to those in the young Taurus region and to sources that have retained their protoplanetary disks in the older Upper Scorpius and η Chamaeleontis stellar clusters, using the same analysis technique for all samples. This comparison allows an investigation of the grain mineralogy evolution with time for a total sample of 139 disks. The mean cluster age and disk fraction are used as indicators of the evolutionary stage of the different populations. Our results show that the disks in the different regions have similar distributions of mean grain sizes and crystallinity fractions (∼10%–20%) despite the spread in mean ages. Furthermore, there is no evidence of preferential grain sizes for any given disk geometry nor for the mean cluster crystallinity fraction to increase with mean age in the 1–8 Myr range. The main implication is that a modest level of crystallinity is established in the disk surface early on (⩽1 Myr), reaching an equilibrium that is independent of what may be happening in the disk midplane. These results are discussed in the context of planet formation, in comparison with mineralogical results from small bodies in our own solar system.

Characterisation of SPH noise in simulations of protoplanetary discs

AbstractThe effects of turbulence on the dynamics of dust grains in protoplanetary discs is of relevant importance in the study of pre-planetesimal formation. The complex interplay between gas and dust and the modelling of turbulence require numerical simulations.A statistical study of the noise in SPH simulations of gas-only protoplanetary accretion discs is performed in order to determine if it could mimic turbulence and to what extent.

C2D Spitzer-IRS spectra of disks around T Tauri stars

Dust grains in the planet forming regions around young stars are expected to be heavily processed due to coagulation, fragmentation and crystallization. This paper focuses on the crystalline silicate dust grains in protoplanetary disks. As part of the Cores to Disks Legacy Program, we obtained more than a hundred Spitzer/IRS spectra of TTauri stars. More than 3/4 of our objects show at least one crystalline silicate emission feature that can be essentially attributed to Mg-rich silicates. Observational properties of the crystalline features seen at lambda > 20 mu correlate with each other, while they are largely uncorrelated with the properties of the amorphous silicate 10 mu feature. This supports the idea that the IRS spectra essentially probe two independent disk regions: a warm zone (< 1 AU) emitting at lambda ~ 10 mu and a much colder region emitting at lambda > 20 mu (< 10 AU). We identify a crystallinity paradox, as the long-wavelength crystalline silicate features are 3.5 times more frequently detected (~55 % vs. ~15%) than the crystalline features arising from much warmer disk regions. This suggests that the disk has an inhomogeneous dust composition within ~10 AU. The abundant crystalline silicates found far from their presumed formation regions suggests efficient outward radial transport mechanisms in the disks. The analysis of the shape and strength of both the amorphous 10 mu feature and the crystalline feature around 23 mu provides evidence for the prevalence of micron-sized grains in upper layers of disks. Their presence in disk atmospheres suggests efficient vertical diffusion, likely accompanied by grain-grain fragmentation to balance the efficient growth expected. Finally, the depletion of submicron-sized grains points toward removal mechanisms such as stellar winds or radiation pressure.

Radiation-pressure mixing of large dust grains in protoplanetary disks

Dusty disks around young stars are formed out of interstellar dust that consists of amorphous, submicrometre grains. Yet the grains found in comets and meteorites, and traced in the spectra of young stars, include large crystalline grains that must have undergone annealing or condensation at temperatures in excess of 1,000 K, even though they are mixed with surrounding material that never experienced temperatures as high as that. This prompted theories of large-scale mixing capable of transporting thermally altered grains from the inner, hot part of accretion disks to outer, colder disk regions, but all have assumptions that may be problematic. Here I report that infrared radiation arising from the dusty disk can loft grains bigger than one micrometre out of the inner disk, whereupon they are pushed outwards by stellar radiation pressure while gliding above the disk. Grains re-enter the disk at radii where it is too cold to produce sufficient infrared radiation-pressure support for a given grain size and solid density. Properties of the observed disks suggest that this process might be active in almost all young stellar objects and young brown dwarfs.

DISK WINDS DRIVEN BY MAGNETOROTATIONAL INSTABILITY AND DISPERSAL OF PROTOPLANETARY DISKS

By performing local three-dimensional MHD simulations of stratified accretion disks, we investigate disk winds driven by MHD turbulence. Initially given weak vertical magnetic fields are effectively amplified by magnetorotational instability and winding due to differential rotation. Large scale channel flows develop most effectively at 1.5 - 2 times the scale heights where the magnetic pressure is comparable to but slightly smaller than the gas pressure. The breakup of these channel flows drives structured disk winds by transporting the Poynting flux to the gas. These features are universally observed in the simulations of various initial fields. This disk wind process should play an essential role in the dynamical evaporation of proto-planetary disks. The breakup of channel flows also excites the momentum fluxes associated with Alfvenic and (magneto-)sonic waves toward the mid-plane, which possibly contribute to the sedimentation of small dust grains in protoplanetary disks.

Magnetorotational Instability in Protoplanetary Disks. II. Ionization State and Unstable Regions

We investigate where magnetorotational instability operates in protoplanetary disks, which can cause angular momentum transport in the disks. We investigate the spatial distribution of various charged particles and the unstable regions for a variety of models for protoplanetary disks, taking into account the recombination of ions and electrons at grain surfaces, which is an important process in most parts of the disks. We find that for all the models there is an inner region that is magnetorotationally stable due to ohmic dissipation. This must make the accretion onto the central star nonsteady. For the model of the minimum-mass solar nebula, the critical radius, inside of which the disk is stable, is about 20 AU, and the mass accretion rate just outside the critical radius is 10-7-10-6 M☉ yr-1. The stable region is smaller in a disk of lower column density. Dust grains in protoplanetary disks may grow by mutual sticking and may sediment toward the midplane of the disks. We find that the stable region shrinks as the grain size increases or the sedimentation proceeds. Therefore, in the late evolutionary stages, protoplanetary disks can be magnetorotationally unstable even in the inner regions.

An Experimental Study on the Structure of Cosmic Dust Aggregates and Their Alignment by Motion Relative to Gas.

We experimentally studied the shape of dust grains grown in a cluster-cluster type of aggregation (CCA) and derived characteristic axial ratios to describe the nonsphericity. CCAs might be described by an axial ratio rhoCCA=rg,max&solm0;rg,min approximately 2.0 in the limit of large aggregates, where rg,min and rg,max describe the minimum and maximum radius of gyration, while small aggregates show a somewhat larger value in their mean axial ratio up to rhoCCA approximately 3.0 but rapidly decrease to the limit rhoCCA approximately 2.0. The axial ratios for large aggregates are in agreement with the general findings of different authors for axial ratios of interstellar dust grains that are generally described by rods or spheroids. Beyond this kind of agreement, our approach does not necessarily require a special shape for individual dust grains but rather offers a physical process to generate nonsphericity. Although the simple shapes might be sufficient for first-order applications and are easier to handle analytically, our results offer a firm ground of special axial ratios for rods or spheroids on a more physical basis apart from any ad hoc assumptions. We also find an alignment of the aggregates during sedimentation in a gas along the drift axis leading to an axial ratio of rhoCCA,align=1.21+/-0.02 with respect to the drift axis and an axis perpendicular to this drift. This result is directly applicable to dust grains in protoplanetary disks and planetary atmospheres.